This application claims the priority of Korean Patent Application No. 2002-68859, filed on Nov. 7, 2002, Korean Patent Application No. 2002-68860, filed Nov. 7, 2002 and Korean Patent Application No. 2002-69418, filed on Nov. 9, 2002, in the Korean Intellectual Property Office, the contents of which are incorporated herein in their entirety by reference.
1. Field of the Invention
The present invention relates to a focus search apparatus, a focus search method, and a focus search waveform generator used in an optical system, and more particularly, to a focus search apparatus and method by which an optical pickup is moved to a focus-on point in response to a signal output from a sine wave generator, after which a focus servo is carried out.
2. Description of the Related Art
FIG. 1 is a block diagram which schematically illustrates a focus servo operation of a general optical system. Referring to FIG. 1, an optical pickup 20 reads data recorded on an optical disc 10. A focus loop filter 30 receives a focus error signal FE output from the optical pickup 20 and outputs a focus output drive signal FOD for a focus servo to an actuator (not shown) mounted in the optical pickup 20 in response to the focus error signal FE. Here, LD and PD denote a laser diode and a photodiode, respectively.
FIG. 2 shows a waveform of a signal output from the focus loop filter 30 shown in FIG. 1. Here, the signal is a triangular wave. The triangular wave is generally generated using an up/down counter (not shown).
A conventional focus search and a focus servo will be described with reference to FIGS. 1 and 2. In order to record or play back a signal on or from the optical disc 10, the optical pickup 20 must be controlled by a focus servo.
If the optical pickup 20 is close to a focused position with respect to the optical disc 10, the optical pickup 20 can generate an S-shaped focus error signal FE. Also, if the optical pickup 20 is positioned within a range corresponding to the range between the maximum value and the minimum value of the S-shaped focus error signal FE, the optical pickup 20 can perform the focus servo.
Accordingly, in order to carry out the focus servo, before the focus servo, the optical pickup 20 has to move to a position within the range in which performing the focus servo is possible. That is, the optical pickup 20 has to move to a position within a range corresponding to the range between the maximum value and the minimum value of the S-shaped focus error signal FE. This operation is referred to as a focus search.
For example, during section a of the curve in FIG. 2, the optical pickup 20 moves away from the optical disc 10 downward. In section b, the optical pickup 20 moves toward the optical disc 10. In section b, the optical pickup 20 performs the focus search and then the focus servo from a focus-on point, i.e., a point in which the focus error signal FE is zero. Thus, a waveform after the focus-on point, i.e., if the focus search succeeds, is a signal for allowing the optical pickup 20 to perform the focus servo.
Referring to FIGS. 1 and 2, since a triangular wave is used in the conventional focus servo, an objective lens (not shown) of the optical pickup 20 vibrates due to a fluctuation in a voltage when performing a focus search. Also, in a case where a voltage difference between the maximum value and the minimum value of a focus error is great and a level of the focus search is low, it is difficult to stably perform a focus servo.
For example, the optical pickup 20 performs a focus search from a focus search start point (SSP) during section b. The optical pickup 20 also performs a focus servo from a focus-on point, i.e., a point in which a focus error signal FE is zero. Thus, a waveform after the focus-on point, i.e., if the focus search succeeds or after a focus pull-in, is a signal for allowing the optical pickup 20 to perform the focus servo.
When carrying out the focus search, i.e., in section b, an optical disc recording apparatus and/or an optical disc reproducing apparatus extract(s) data on the optical disc 10, e.g., data on the type of a laser disc (LD), a compact disc (CD), and a digital versatile disc (DVD), or data on whether a disc exists.
However, in an event that a focus drop occurs due to an external impact when performing the focus search, a focus search apparatus performs the focus search again from the start.
Referring to FIG. 2, in a case where the focus search apparatus performs the focus search again, the focus search apparatus starts the focus search from the SSP. Thus, it takes a considerable amount of time to perform the focus search from the start. Also, in a case where the focus search is performed using a triangular wave, if the speed for the focus search increases, an objective lens (not shown) of the optical pickup 20 vibrates. If the speed of focus search decreases to prevent vibration of the optical pickup 20, performing the focus search after a focus drop requires a significant amount of time.
In general, an optical system, i.e., an optical disc recording apparatus and/or an optical disc reproducing apparatus, records or plays back data on or from an optical disc (or an optical recording medium), such as a CD, a DVD, or the like.
FIG. 3 is a schematic block diagram of a conventional optical disc reproducing apparatus. Referring to FIG. 3, an optical disc reproducing apparatus 300 includes an optical disc 310, a spindle motor 320, an optical pickup 330, a focusing servo 340, a tracking servo 350, a spindle servo 360, and a controller 370.
The optical pickup 330 irradiates a laser beam onto the optical disc 310 and outputs a data signal, a focus error signal, and a tracking error signal according to the laser beam reflected from the optical disc 310. The focusing servo 340 detects the focus error signal and drives an objective lens (not shown) of the optical pickup 330 within the range of the depth of a focus in response to the focus error signal. The tracking servo 350 detects the tracking error signal and follows a track in response to the tracking error signal.
The spindle servo 360 controls the spindle motor 320 that spins the optical disc 310. The controller 370 controls the over-all operations of the focusing servo 340, the tracking servo 350, and the spindle servo 360.
In order to stably carry out an optimum tracking servo and/or a focus servo, an optical disc reproducing apparatus performing the above-described operations has to control gain of a servo system, i.e., a tracking servo and/or a focusing servo.
Also, in light of the tendency toward compact and light optical disc reproducing devices, the servo system and an optical disc reproducing apparatus using the servo system have to be light and consume a small amount of power.